At the present time, virtually all printed copy is produced through the use of three basic types of printing plates. One type is a relief plate which prints from a raised surface. Another type is an intaglio plate which prints from a depressed surface. The third type is the lithographic plate which prints from a substantially flat surface which is neither appreciably raised above nor appreciably depressed below the adjacent and surrounding non-printing areas. Printing is occasioned by an ink's respective affinity and/or aversion to areas of different chemical properties. Lithographic printing plates are commonly processed to have water-repellent (hydrophobic), oil-receptive (oleophilic) image areas and water-receptive (hydrophilic) non-image areas.
Prior to processing for use, conventional lithographic plates will typically have a hydrophobic, photoreactive polymeric layer (i.e. photoresist) coated or otherwise deposited atop a hydrophilic substrate.
In preparing a conventional lithographic plate for use on a printing press, the plate is first exposed to actinic radiation. Specific chemical reactions are caused to occur in the plate's photoresist by exposure to actinic radiation. Such photoinduced chemical reactions may either reduce or enhance the solubility of the photoresist, depending on whether the resist is negative-working or positive- working. In negative-working plates, exposure to actinic radiation will generally cause a "hardening" of the photoresist. In positive-working plates, exposure to actinic radiation will generally cause a "softening" or solubilization of the photoresist.
After photoexposure, a wet development step is normally conducted. The objective of such wet development is to remove those areas of the photoresist which have undergone photoinduced chemical change or those which have not been photoexposed. Solvation under conventional development techniques will typically involve treating the exposed plate with organic solvents in a developing bath. For negative-working resists, the solvent will swell and dissolve the unexposed portions of the resist. The solvent should not swell the exposed portions or distortion of the developed image may result. For positive-working resists, the response of the unexposed and exposed coatings are reversed, but the same general principles apply.
As a result of the preferential solvation and washing away of portions of the photoresist, corresponding portions of the underlying hydrophilic substrate are uncovered. For negative-working plates, the aforementioned hydrophobic image areas correspond to the portions of the photoresist remaining after solvation and washing. The aforementioned hydrophilic non-image areas correspond to uncovered portions of the substrate. The image and non-image areas thus differentiated, the processed plate may then be mounted onto a printing press and run.
Encumbered by required wet development, the processing of conventional solvent-based lithographic plates prior to their use on a printing press is time and labor consuming and involves the use of substantial quantities of organic chemicals. It will be appreciated that there is considerable attractiveness for innovations that would satisfactorily eliminate or reduce conventional lithography's long-felt dependency upon the conduct of wet development and thereby permit the use of solvent-based lithographic plates on a printing press immediately after exposure without required intermediary processing.
In the past, dry developable lithographic printing plates have been suggested which enable the wet processing steps of lithographic printing plates after exposure to be omitted and printing to be conducted by directly mounting the exposed plates on a printing press. Among printing plates that may be characterized as on-press developable (or related thereto) are: e.g., U.S. Pat. No. 4,273,851, issued to Muzyczko et al. on Jun. 16, 1981; U.S. Pat. No. 5,258,263, issued to Z. K. Cheema, A. C. Giudice, E. L. Langlais, and C. F. St. Jacques on Nov. 2, 1993; and U.S. Pat. No. 5,395,734, issued to Vogel et al. on Mar. 7, 1995.
Despite the methodologies and approaches embodied in the aforementioned patents, there is a continuing need for a lithographic printing plate that can be readily developed on a printing press and that produces a plate having durable image areas needed for good run length. Difficulty in the realization simultaneously of both "on-press developability" and "durability" is believed to originate from an apparent contradiction between photoresist removability ("developability") on the one hand and "durability" on the other: To make a photoresist more durable was to make the photoresist less developable.
In designing an on-press developable plate, the goals of "developability" and "durability" are often competing. For example, it has been found that good developability may be achieved by increasing the affinity of the resist to fountain and ink solutions, thus promoting a faster rate of penetration of such press solutions into the photoresist. Alternatively, it has also been found that good developability may be achieved by increasing the volume of the photoresist composition deposited above the hydrophilic substrate. Regardless, it has been observed that both modifications, standing alone, invariably tend to increase the tackiness of the printing plate, and thus, reduce its durability.
While it is possible to more closely align the goals of "durability" and "developability" by the utilization of comparatively greater concentrations of binder compositions capable of effectively serving as both a matrix and a photoreactive component (see, U.S. Pat. No. 5,514,522 and/or by the use of fountain leachable plasticizers (see, U.S. pat. app. Ser. No. 08/147,044, cross-referenced above), an additional means has been found which is capable of promoting "durability" without effecting significant losses of "developability". Gains in resolution have also been observed.
More specifically, it has been found that dispersed hydrophobic rubber components may be effectively incorporated by the use of surfactants into a photoresist to durabilize and detackify the photoresist, thereby making such photoresist more suitable for on-press use. While rubbers have been disclosed for use in conventional printing plates, the use of rubber materials to advance the goals of on-press developability has not been foreshown. On-press developability imposes certain limitations that would belie obvious translation of conventional uses. Unlike conventional lithography wherein there exists greater latitude for the selection and conduct of development materials and processes, on-press development techniques are in great part constrained by the several process parameters necessary to achieve satisfactory on-press development, such as the use of "weaker" developers and compatibility with the printing press environment. Although these parameters vary among the different on-press modalities, the character, configuration and function of the developers are typically of great concern. Accordingly, the use of hydrophobic rubber additives in an on-press system was counterintuitive under conventional teachings, since to make the resist more hydrophobic would have been to narrow an already narrowly constrained means of development. Accordingly, no known on-press technologies have incorporated rubber as a means to enhance the durability of photoresists.
Departing from prevailing ideas, the present invention is able to strike a balance between the competing goals of durability and developability in a system wherein rubber additives are incorporated to effectively enhance the durability of a photoresist, but configured to minimize interference with potential on-press development systems. Briefly, this is preferably accomplished by incorporating rubber additives as discrete particles stably suspended in the photoresist by the agency of suitable surfactants. Lithographic printing plates utilizing the dispersed particulate rubber system manifests good durability, developability and resolution.
While applicants do not wish to be bound to any particular mechanism in explaining the functionality of the preferred dispersed particulate rubber system, it is believed that a number of factors are involved. Briefly, durability is effected by improving adhesivity and resistance to external stress; developability is effected by the dispersed and particulate configuration of the rubber; and resolution was effected by rubber's hydrophobicity.
With regard to durability, it is noted that the durability of a lithographic printing plate, (i.e., the capacity of a printing plate to be run continuously on a printing press without significant loss of resolution), is positively correlated with adhesivity of the photoresist to its underlying substrate: i.e., if the resist is well adhered to the substrate, the printing plate can maintain longer press runs. Good adhesion is best achieved when internal stress in the photoresist is reduced to a minimum level. However, high levels of internal stress are generated when the photoresist shrinks due to the extremely fast rate of monomer polymerization relative to the rate of relaxation of crosslinked binders. Shrinkage is especially pronounced in photoresists that utilize lesser concentration of binder as a trade-off for enhanced developability.
The incorporation of discrete rubber particles in the photoresist reduces shrinkage. During polymerization, the dispersed rubber particles are believed to expand as a result of temperature increases and swelling caused by absorption of unreacted monomers. The expansion of the particulate rubber phase is believed to compensate the shrinkage of the surrounding continuous resist phase. In designs utilizing solvent-soluble, phase-separable rubbery additives (discussed below), compensation is accomplished when the rubbery materials separate from its polymer matrix to form discrete domains.
Aside from compensating for shrinkage, the durability of a printing plate is also believed to be enhanced by virtue of the mechanical properties of rubber, e.g., compressibility, flexibility and elasticity. When run on a printing press, the compressive and rotational forces generated by the press rollers tend to disfigure, erode or otherwise damage the surface of the plate and any coatings deposited thereon. By the use of dispersed rubber additives, it is believed that crazing and/or shear banding mechanisms are promoted, thus providing avenues for the non-disruptive release of externally imposed stresses.
With the capacity to enhance durability, dispersed particulate rubber materials make practical the use of thinner photoresists that are more permeable to developers and press solutions. Regardless, even without modifying photoresist thickness, the preferred embodiment of the present invention effectively maintains the developability of the photoresist by virtue of the particular configuration of the rubber additives. Both dispersed and particulate in character, the rubber additives offer minimal interference with the penetration of developers and fountain solutions.
Finally, with regard to resolution, the incorporation of rubber into a photoresist significantly raises its hydrophobicity. Greater levels of hydrophobicity translate into greater ink transfer efficiency from the plate to a receiving medium, such as paper. Together with the enhanced durability of exposed highlight areas, the greater ink transfer efficiency increases the resolution of the printing plate.